ASTM D6820-20
(Guide)Standard Guide for Use of the Time Domain Electromagnetic Method for Geophysical Subsurface Site Investigation
Standard Guide for Use of the Time Domain Electromagnetic Method for Geophysical Subsurface Site Investigation
SIGNIFICANCE AND USE
5.1 Concepts:
5.1.1 All TDEM/TEM instruments are based on the concept that a time-varying magnetic field generated by a change in the current flowing in a large loop on the ground will cause current to flow in the earth below it (Fig. 3). In the typical TDEM/TEM system, these earth-induced currents are generated by abruptly terminating a steady current flowing in the transmitter loop (2). The currents induced in the earth material move downward and outward with time and, in a horizontally layered earth, the strength of the currents is directly related to the ground conductivity at that depth. These currents decay exponentially. The decay lasts microseconds, except in the cases of a highly conductive ore body or conductive layer when the decay can last up to a second. Hence, many measurements can be made in a short time period allowing the data quality to be improved by stacking.
5.1.2 Most TDEM/TEM systems use a square wave transmitter current with the measurements taken during the off-time (Fig. 2) with the total measurement period of less than a minute. Because the strength of the signal depends on the induced current strength and secondary magnetic field, the depth of site investigation depends on the magnetic moment of the transmitter.
5.1.3 A typical transient response, or receiver voltage measured, for a homogeneous subsurface (half-space) is shown in Fig. 4. The resistivity of the subsurface is obtained from the late stage response. If there are two horizontal layers with different resistivities, the response or receiver output voltage is similar to the curves shown in Fig. 5.
5.2.8 Variations in temperature above freezing will affect resistivity measurements as a result of the temperature dependence of the resistivity of the pore fluid, which is of the order of 2 % per degree Celsius (1 % per degree Fahrenheit). Thus, data from measurements made in winter can be quite different from those made in summer.
5.2.9 As the ground temperature decre...
SCOPE
1.1 Purpose and Application:
1.1.1 This guide is one in a series of documents that describe geophysical site investigation methods.
1.1.2 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of subsurface materials and their pore fluids using the Time Domain Electromagnetic (TDEM) method. This method is also known as the Transient Electromagnetic (TEM) Method, and in this guide is referred to as the TDEM/TEM method. Time Domain and Transient refer to the measurement of a time-varying induced electromagnetic field.
1.1.3 The TDEM/TEM method is applicable to the subsurface site investigation for a wide range of conditions. TDEM/TEM methods measure variations in the electrical resistivity (or the reciprocal, the electrical conductivity) of the subsurface soil or rock caused by both lateral and vertical variations in various physical properties of the soil or rock. By measuring both lateral and vertical changes in resistivity, variations in subsurface conditions can be determined.
1.1.4 Electromagnetic measurements of resistivity as described in this guide are applied in geologic studies, geotechnical studies, hydrologic site investigations, and for mapping subsurface conditions at waste disposal sites (1).2 Resistivity measurements can be used to map geologic changes such as lithology, geological structure, fractures, stratigraphy, and depth to bedrock. In addition, measurement of resistivity can be applied to hydrologic site investigations such as the depth to water table, depth to aquitard, presence of coastal or inland groundwater salinity, and for the direct exploration for groundwater.
1.1.5 This standard does not address the use of TDEM/TEM method for use as metal detectors or their use in unexploded ordnance (UXO) detection and characterization. While many of the principles apply the data acquisition and interpretation differ from those set forth in this...
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This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation:D6820 −20
Standard Guide for
Use of the Time Domain Electromagnetic Method for
1
Geophysical Subsurface Site Investigation
This standard is issued under the fixed designation D6820; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope of the principles apply the data acquisition and interpretation
differ from those set forth in this standard guide.
1.1 Purpose and Application:
1.1.6 General references for the use of the method are
1.1.1 This guide is one in a series of documents that
McNeill (2), Kearey and Brooks (3), and Telford et al (4).
describe geophysical site investigation methods.
1.1.2 This guide summarizes the equipment, field
1.2 Limitations:
procedures, and interpretation methods for the assessment of
1.2.1 This guide provides an overview of the TDEM/TEM
subsurface materials and their pore fluids using the Time
method.Itdoesnotprovideoraddressthedetailsofthetheory,
DomainElectromagnetic(TDEM)method.Thismethodisalso
field procedures, or interpretation of the data. Numerous
knownastheTransientElectromagnetic(TEM)Method,andin
references are included for that purpose and are considered an
this guide is referred to as the TDEM/TEM method. Time
essential part of this guide. It is recommended that the user of
Domain and Transient refer to the measurement of a time-
the TDEM/TEM method be familiar with the references cited
varying induced electromagnetic field.
and with the ASTM standards D420, D653, D5088, D5608,
1.1.3 The TDEM/TEM method is applicable to the subsur-
D5730, D5753, D6235, D6429 and D6431.
face site investigation for a wide range of conditions. TDEM/
1.2.2 This guide is limited to TDEM/TEM measurements
TEM methods measure variations in the electrical resistivity
made on land. The TDEM/TEM method can be adapted for a
(orthereciprocal,theelectricalconductivity)ofthesubsurface
number of special uses on land, water, ice, within a borehole,
soil or rock caused by both lateral and vertical variations in
and airborne. Special TDEM/TEM configurations are used for
various physical properties of the soil or rock. By measuring
metal and unexploded ordnance detection. These TDEM/TEM
both lateral and vertical changes in resistivity, variations in
methods are not discussed in this guide.
subsurface conditions can be determined.
1.2.3 TheapproachessuggestedinthisguidefortheTDEM/
1.1.4 Electromagnetic measurements of resistivity as de-
TEM method are commonly used, widely accepted, and
scribed in this guide are applied in geologic studies, geotech-
proven. However, other approaches or modifications to the
nical studies, hydrologic site investigations, and for mapping
2
TDEM/TEM method that are technically sound may be sub-
subsurface conditions at waste disposal sites (1). Resistivity
stituted.
measurements can be used to map geologic changes such as
1.2.4 This guide offers an organized collection of informa-
lithology, geological structure, fractures, stratigraphy, and
tion or a series of options and does not recommend a specific
depth to bedrock. In addition, measurement of resistivity can
course of action. This document cannot replace education,
beappliedtohydrologicsiteinvestigationssuchasthedepthto
experience, and should be used in conjunction with profes-
water table, depth to aquitard, presence of coastal or inland
sional judgment. Not all aspects of this guide may be appli-
groundwatersalinity,andforthedirectexplorationforground-
cable in all circumstances. ThisASTM standard is not intended
water.
to represent or replace the standard of care by which the
1.1.5 ThisstandarddoesnotaddresstheuseofTDEM/TEM
adequacy of a given professional service must be judged, nor
method for use as metal detectors or their use in unexploded
should this document be applied without consideration of a
ordnance (UXO) detection and characterization. While many
project’smanyuniqueaspects.Thewordstandardinthetitleof
this document means only that the document has been ap-
1 proved through the ASTM consensus process.
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
1.3 Precautions:
Characterization.
Current edition approved Jan. 1, 2020. Published January 2020. Originally
1.3.1 It is the responsibility of the user of this guide to
approved in 2002. Last previous edition approved in 2018 as D6820–18. DOI:
follow any precautions in the equipment manufacturer’s
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6820 − 18 D6820 − 20
Standard Guide for
Use of the Time Domain Electromagnetic Method for
1
Geophysical Subsurface Site CharacterizationInvestigation
This standard is issued under the fixed designation D6820; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*Scope
1.1 Purpose and Application:
1.1.1 This guide is one in a series of documents that describe geophysical site investigation methods.
1.1.2 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of subsurface
materials and their pore fluids using the Time Domain Electromagnetic (TDEM) method. This method is also known as the
Transient Electromagnetic (TEM) Method, and in this guide is referred to as the TDEM/TEM method. Time Domain and Transient
refer to the measurement of a time-varying induced electromagnetic field.
1.1.3 The TDEM/TEM method is applicable to the subsurface site characterizationinvestigation for a wide range of conditions.
TDEM/TEM methods measure variations in the electrical resistivity (or the reciprocal, the electrical conductivity) of the subsurface
soil or rock caused by both lateral and vertical variations in various physical properties of the soil or rock. By measuring both
lateral and vertical changes in resistivity, variations in subsurface conditions can be determined.
1.1.4 Electromagnetic measurements of resistivity as described in this guide are applied in geologic studies, geotechnical
2
studies, hydrologic site characterizations,investigations, and for mapping subsurface conditions at waste disposal sites (1).
Resistivity measurements can be used to map geologic changes such as lithology, geological structure, fractures, stratigraphy, and
depth to bedrock. In addition, measurement of resistivity can be applied to hydrologic site characterizationsinvestigations such as
the depth to water table, depth to aquitard, presence of coastal or inland groundwater salinity, and for the direct exploration for
groundwater.
1.1.5 This standard does not address the use of TDEM/TEM method for use as metal detectors or their use in unexploded
ordnance (UXO) detection and characterization. While many of the principles apply the data acquisition and interpretation differ
from those set forth in this standard guide.
1.1.6 General references for the use of the method are McNeill (2), Kearey and Brooks (3), and Telford et al (4).
1.2 Limitations:
1.2.1 This guide provides an overview of the TDEM/TEM method. It does not provide or address the details of the theory, field
procedures, or interpretation of the data. Numerous references are included for that purpose and are considered an essential part
of this guide. It is recommended that the user of the TDEM/TEM method be familiar with the references cited and with the ASTM
standards D420, D653, D5088, D5608, D5730, D5753, D6235, D6429 and D6431.
1.2.2 This guide is limited to TDEM/TEM measurements made on land. The TDEM/TEM method can be adapted for a number
of special uses on land, water, ice, within a borehole, and airborne. Special TDEM/TEM configurations are used for metal and
unexploded ordnance detection. These TDEM/TEM methods are not discussed in this guide.
1.2.3 The approaches suggested in this guide for the TDEM/TEM method are commonly used, widely accepted, and proven.
However, other approaches or modifications to the TDEM/TEM method that are technically sound may be substituted.
1.2.4 This guide offers an organized collection of information or a series of options and does not recommend a specific course
of action. This document cannot replace education, experience, and should be used in conjunction with professional judgment. Not
all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the
standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied
without consideration of a project’s many unique aspects. The word standard in the title of this document means only that the
document has been approved through the ASTM consensus process.
1
This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
...
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